Radio-frequency (rf) to direct current (dc) converter and bipolar quantized supercurrent generator (qsg)

ABSTRACT

A radio-frequency (RF) to direct current (DC) converter is provided. When a DC electrical current is applied via a DC input port of the converter, the DC electrical current is shunted to ground through a Josephson junction (JJ) of the converter and substantially no DC electrical current flows through a resistor of the converter, and when an RF electrical current is applied via an RF input port of the converter, output trains of SFQ current pulses from a DC to SFQ converter of the RF-to-DC converter with pulse-to-pulse spacing inversely proportional to the RF electrical current frequency cause the JJ to switch at a rate commensurate with an RF frequency of the RF electrical current to generate a steady state voltage across the JJ linearly dependent on the RF frequency.

BACKGROUND

The currently claimed embodiments of the present invention relate toquantum computation, and more specifically, to a radio-frequency (RF) todirect current (DC) converter and bipolar quantized supercurrentgenerator (QSG) and quantum mechanical systems using the same.

The application of fast flux bias pulses has traditionally beenaccomplished by room temperature giga samples per second (GS/s)digital-to-analog converters (DACs) driving current into temperatureT=4K resistors with the precision of the pulse amplitude set by a numberof DAC bits and the Johnson noise of the bias resistor. However, thismethod suffers from large pulse shape distortion as the current pulsemust traverse multiple temperature stages and filtering before arrivingat the intended device (device under test—DUT). Additionally, to achieveprecise pulse heights, a voltage DAC with a relatively large count isrequired. Furthermore, the wiring required to bias a plurality n ofdevices scales linearly with the number n of the plurality of devices.Therefore, it is desirable to provide a new way or system to generatefast current pulses that can either be applied directly to the device(DUT) or coupled as a flux via a pair of mutual inductors.

In addition, the application of static flux bias to superconductingcircuitry has predominantly been performed via the application of avoltage to a cold resistor (e.g., at a temperature T of about 4K) whichthen drives current to a primary inductance loop mutually coupled to adevice (DUT). The scaling of this approach in terms of the roomtemperature wiring and voltage source overhead is linear in the numberof devices (DUTs) one wishes to flux bias. The heat load and physicalspace required to accommodate hundreds if not thousands of physicaldevices (e.g., qubits) required to demonstrate quantum advantage viaimplementation of computing paradigms such as the surface code isuntenable. Therefore, it is also desirable to provide a new way orsystem to generate bi-polar flux bias current such that the scaling innumber of devices to room temperature control lines is improved whileachieving minimal to zero dynamic heat load at any stage of thecryostat.

SUMMARY

An aspect of the present invention is to provide a radio-frequency (RF)to direct current (DC) converter including a direct electrical current(DC) input port; a radiofrequency (RF) input port; and a directelectrical current (DC) to single flux quantum (SFQ) converter connectedto the RF input port, the DC to SFQ converter being configured toconvert RF current to SFQ current pulses. The converter further includesa Josephson junction (JJ) connected to both the DC input port via afirst induction line and to the DC to SFQ converter via a secondinduction line and to ground; and a resistor connected to the Josephsonjunction and to the DC input port via a third induction line. Inoperation, when a DC electrical current is applied via the DC inputport, the DC electrical current is shunted to ground through the JJ andsubstantially no DC electrical current flows through the resistor, andwhen an RF electrical current is applied via the RF input port, outputtrains of SFQ current pulses from the DC to SFQ converter withpulse-to-pulse spacing inversely proportional to the RF electricalcurrent frequency cause the Josephson junction (JJ) to switch at a ratecommensurate with an RF frequency of the RF electrical current togenerate a steady state voltage across the Josephson junction (JJ)linearly dependent on the RF frequency such that an electrical currentflowing through the resistor is directly dependent on the RF frequencyof the RF electrical current.

In an embodiment, the converter further includes a plurality ofJosephson junctions connected to both the DC input port via the firstinduction line and to the RF input port via the DC to SFQ converterthrough the second induction line and to ground. The plurality ofJosephson junctions are configured to switch, when the RF electricalcurrent is applied via the RF input port, at a rate commensurate the RFfrequency of the RF electrical current to generate a steady statevoltage across the plurality of Josephson junctions (JJ) linearlydependent on the RF frequency such that the electrical current flowingthrough the resistor is directly dependent on the RF frequency of the RFelectrical current.

In an embodiment, the steady state voltage (V) across the Josephsonjunction (JJ) is proportional to the RF frequency (f_(clk)) according tothe following equation: V=Φ₀×f_(clk), where Φ₀ is a superconductingmagnetic flux quantum.

A further aspect of the present invention is to provide a quantummechanical system including the above radio-frequency (RF) to directcurrent (DC) converter. In an embodiment, the quantum mechanical systemfurther includes at least one quantum mechanical device connected to theresistor. In an embodiment, the one or more devices includes, forexample, a qubit, a superconducting quantum interference device, or anon-quantum mechanical circuit.

In an embodiment, the quantum mechanical system further includes a rapidsingle flux quantum (RSFQ) pulse doubler having an input port and anoutput port. The output port of the RSFQ pulse doubler is connected toan input port of the DC to SFQ converter, the RSFQ pulse doubler beingconfigured to generate SFQ pulse current input through the input port ofthe DC to SFQ converter.

In an embodiment, the RSFQ pulse doubler is configured to generate aplurality of current pulses at double a rate of applied SFQ pulses atthe output port of the RSFQ pulse doubler from a single radiofrequencycurrent pulse input at the input port of RSFQ pulse doubler so as togenerate a larger voltage across the converter.

In an embodiment, the quantum mechanical system further includes mstages of the rapid single flux quantum (RSFQ) pulse doubler connectedin series. The m stages of the RSFQ pulse doubler are configured togenerate the RF electrical current (I) that is given by the followingformula: I=2^(m)×Φ₀×f_(clk)/R, where Φ₀ is a superconducting magneticflux quantum, f_(clk) is the RF frequency of the RF electrical current,and R is a resistance value of the resistor.

In an embodiment, the quantum mechanical system includes a plurality ofradio-frequency (RF) to direct current (DC) converters; and a pluralityof quantum mechanical devices, each of the plurality of quantummechanical devices being connected to a corresponding one of theplurality of RF to DC converters. The plurality of RF to DC convertersare addressable so that the SFQ pulses produced from the RF electricalcurrent are routed to a desired converter in the plurality of RF to DCconverters.

In an embodiment, the quantum mechanical system further includes aninput port configured to receive the direct electrical current (DC) andthe radiofrequency (RF) electrical current; and a plurality of addresslines, each address line having at least one demultiplexer (DEMUX) and ademultiplexer (DEMUX) in a first address line is connected to the inputport. Each converter in the plurality of radio-frequency (RF) to directcurrent (DC) converters is connected to a corresponding one of the leastone demultiplexer (DEMUX).

In an embodiment, the demultiplexer (DEMUX) in the first address line isconnected to two demultiplexers (DEMUX) in a second address line andeach of the two demultiplexers is connected to at least tworadio-frequency (RF) to direct current (DC) converters.

Another aspect of the present invention is to provide a bipolarquantized supercurrent generator (QSG) including a first input portconfigured to receive at least one increment single flux quantum pulsesand a second input port configured to receive at least one decrementsingle flux quantum pulses. The QSG further includes a first Josephsonjunction (JJ) connected to the first input port and a second Josephsonjunction (JJ) connected to the second input port, the first and secondJosephson junctions being further connected to ground; and an inductor(L_(q)) connected to the first Josephson junction and to the secondJosephson junction. The inductor (L_(q)) and the first and secondJosephson junctions (JJs) form a superconducting quantum interferencedevice (SQUID) loop. In operation, an electrical current circulating inthe storage SQUID loop formed by the first Josephson junction, thesecond Josephson junction and the inductor (L_(q)) increases ordecreases in increments based on the at least one increment single fluxquantum pulses input through the first input port or the at least onedecrement single flux quantum pulses input through the second inputport, respectively.

In an embodiment, the electrical current circulating in the storageSQUID loop increases or decreases by electrical current increments AIgiven by the following equation: ΔI=Φ₀/L_(q), where Φ₀ is asuperconducting magnetic flux quantum and L_(q) is an inductance valueof the inductor (L_(q)).

In an embodiment, the QSG further includes a third input port connectedto the first Josephson junction (JJ), the second Josephson junction (JJ)and the inductor (Lq), the third input port being configured to input abias direct electrical current (DC) to the storage loop to electricallybias the first and second Josephson junctions so that the first andsecond Josephson junctions produce pulses when pulses are applied totheir respective inputs.

In an embodiment, a screening parameter β_(L) of the SQUID loop isdependent on a critical electrical current I of the first and secondJosephson junctions in the SQUID loop and the inductance value of theinductor connecting the first and second Josephson junctions.

In an embodiment, the QSG further includes a third Josephson junction(JJ) connected to the first Josephson junction (JJ) and the first inputport via a first induction line; and a fourth Josephson junction (JJ)connected to the second Josephson junction (JJ) and the second inputport via a second induction line. The first Josephson junction (JJ) andthe third Josephson junction (JJ) form a first Josephson transmissionline (JTL) and the second Josephson junction (JJ) and the fourthJosephson junction (JJ) form a second Josephson transmission line (JTL).

Yet another aspect of the present invention is to provide a quantummechanical system comprising the above QSG. In an embodiment, thequantum mechanical system further includes a plurality of bipolarquantized supercurrent generators (QSGs); and a plurality of quantummechanical devices, each of the plurality of quantum mechanical devicesbeing inductively coupled to a corresponding one of the plurality ofQSGs. The plurality of QSGs are addressable so that input SFQ pulses arerouted to a desired QSG in the plurality of QSGs.

In an embodiment, the quantum mechanical system further includes aninput port configured to receive a direct electrical current (DC) and asingle flux quantum (SFQ) radiofrequency current; and a plurality ofaddress lines, each address line having at least one demultiplexer(DEMUX) a demultiplexer (DEMUX) in a first address line is connected tothe input port. Each QSG is connected to a corresponding one of theleast one demultiplexer (DEMUX).

BRIEF DESCRIPTION OF THE DRAWINGS

The present disclosure, as well as the methods of operation andfunctions of the related elements of structure and the combination ofparts and economies of manufacture, will become more apparent uponconsideration of the following description and the appended claims withreference to the accompanying drawings, all of which form a part of thisspecification, wherein like reference numerals designate correspondingparts in the various figures. It is to be expressly understood, however,that the drawings are for the purpose of illustration and descriptiononly and are not intended as a definition of the limits of theinvention.

FIG. 1 is a schematic electronic circuit of a radio-frequency (RF) todirect current (DC) converter, according to an embodiment of the presentinvention;

FIG. 2 is a plot of a resultant Current-Voltage for a simulation (forexample, using WRSpice) of a 20 JJs (e.g., feeding Josephsontransmission line—FJTL) for different drive frequencies (frequenciesshown are 5 GHz, 7GHz, 9 GHz, 11 GHz, 13 GHz, and 15 GHz), according toan embodiment of the present invention;

FIG. 3 is a plot of an output resistor current (in μA) versus time (inns) obtained from a dynamic simulation (for example, using WRSpice) ofcurrent driven through a series combination of a 130 pH inductor and 0.1resistor from a 20 JJ FJTL as a function of RF drive frequency,according to an embodiment of the present invention;

FIG. 4 is a block diagram of a quantum mechanical system including theRF to DC converter shown in FIG. 1, according to an embodiment of thepresent invention;

FIG. 5 is a schematic electronic circuit of an example conventionalrapid single flux quantum (RSFQ) pulse doubler;

FIG. 6 is a plot of a single pulse input into a series array of fivecascaded RSFQ pulse doublers and a plurality of pulses output by theRSFQ pulse doublers, according to an embodiment of the presentinvention;

FIG. 7 is schematic diagram depicting how a single DC/SFQ converter 106can provide controllably fast flux bias to a plurality of devices (DUTs)via a plurality of RF to DC converters, according to an embodiment ofthe present invention;

FIG. 8 is a schematic electronic circuit of a bipolar quantizedsupercurrent generator (QSG), according to an embodiment of the presentinvention;

FIGS. 9A-9D show dynamic simulations of the bipolar quantizedsupercurrent generator (QSG) shown in FIG. 8, according to an embodimentof the present invention; and

FIG. 10 is a schematic diagram of a quantum mechanical system using theQSG shown in FIG. 8, according to an embodiment of the presentinvention.

DETAILED DESCRIPTION

FIG. 1 is a schematic electronic circuit of a radio-frequency (RF) todirect current (DC) converter, according to an embodiment of the presentinvention. The radio-frequency (RF) to direct current (DC) converter 100includes a direct electrical current (DC) input port 102 and aradiofrequency (RF) input port 104. The converter 100 also includes adirect electrical current (DC) to single flux quantum (SFQ) (DC/SFQ)converter 106 connected to the RF input port 104. The DC to SFQconverter 106 is configured to convert RF current to SFQ current pulses.The radio-frequency (RF) to direct current (DC) converter 100 alsoincludes a Josephson junction (JJ) 108A connected to both the DC inputport 102 via a first induction line 110 and to the DC to SFQ converter106 via a second induction line 112 and to ground 114. The converter 100also includes a resistor 118 connected to the Josephson junction (JJ)108A and to the DC input port 102 via a third induction line 120.

In operation, when a DC electrical current is applied via the DC inputport 102, the DC electrical current is shunted to ground 116 through theJJ 108A and substantially no DC electrical current flows through theresistor 118, and when an RF electrical current is applied via the RFinput port 104, output trains of SFQ current pulses from the DC to SFQconverter 106 with pulse-to-pulse spacing inversely proportional to theRF electrical current frequency cause the Josephson junction (JJ) 108Ato switch at a rate commensurate with an RF frequency of the RFelectrical current to generate a steady state voltage across theJosephson junction (JJ) 108A linearly dependent on the RF frequency suchthat an electrical current flowing through the resistor 118 is directlydependent on the RF frequency of the RF electrical current.

In an embodiment, the converter 100 further includes a plurality ofJosephson junctions 108A, 108B, 108C, 108D connected to both the DCinput port 102 via the first induction line 110 and to the RF input port104 via the DC to SFQ converter 106 through the second induction line112 and to ground 116. Although four Josephson junctions are depicted inFIG. 1, as it can be appreciated any number of Josephson junctions canbe used, for example, two, three or more. The plurality of Josephsonjunctions 108A, 108B, 108C, 108D are configured to switch, when the RFelectrical current is applied via the RF input port 104, at a ratecommensurate the RF frequency of the RF electrical current to generate asteady state voltage across the plurality of Josephson junctions (JJ)108A, 108B, 108C, 108D linearly dependent on the RF frequency such thatthe electrical current flowing through the resistor 118 is directlydependent on the RF frequency of the RF electrical current. It might beworth noting that the more JJs used in the FJTL, the greater the amountof current the FJTL can source was driven with RF.

As shown in FIG. 1, when a global bias current is applied at DC inputport 102, the current is completely shunted to ground (up arrows)through the JJs 108A, 108B, 108C and 108D and no current flows (downarrows) through the resistive path (i.e., resistance 118) towards adevice (DUT) (not shown). When an RF current is applied at the RF inputport 104, the JJs 108A, 108B, 108C and 108D begin to switch at a ratecommensurate with the frequency of the applied RF tone and develop asteady state voltage linearly dependent on the drive frequency. The FJTLdevelops a steady state DC voltage (V) across the Josephson junctions(JJ) 108A, 108B, 108C, 108D that is proportional to the RF frequency(folk) according to the following equation:

V=Φ ₀ ×f _(clk)

where Φ₀ is a superconducting magnetic flux quantum.

This voltage then drives a current through the resistor 118 where themaginitude of the current is given by Ohm's law I=V/R=Φ₀×f_(clk)/R .Thus, the last relation shows that this circuit is a true RF frequencyto DC current converter where the linear scaling is set by the magneticflux quantum and the shunt resistor.

FIG. 2 is a plot of a resultant Current-Voltage for a simulation (forexample, using WRSpice) of a 20 JJs (e.g., feeding Josephsontransmission line—FJTL) for different drive frequencies (frequenciesshown are 5 GHz, 7GHz, 9 GHz, 11 GHz, 13 GHz, and 15 GHz), according toan embodiment of the present invention. In an embodiment, all the JJshad a critical current of 250 μA resulting in a total critical currentof the circuit of 5 mA. FIG. 2 shows Shapiro steps begin atapproximately 1.5 mA of applied DC bias to the FJTL for all frequenciesapplied. When globally biased at a current of about 3.5 mA, for example,the FJTL can source or sink up to about 1.5 mA of current and remainoperational. The voltage developed across the FJTL is extremely stablefor a large range of global bias current.

FIG. 3 is a plot of the output resistor current (in μA) versus time (inns) obtained from a dynamic simulation (for example, using WRSpice) ofcurrent driven through a series combination of a 130 pH inductor and 0.1resistor from a 20 JJ FJTL as a function of RF drive frequency,according to an embodiment of the present invention. WRSpice is acircuit simulation and analysis tool produced Whiteley ResearchIncorporated. At time t=0 ns, an RF current of 5 GHz is applied to a thedc/SFQ converter 106. The output pulses from the converter 106 are thenfed into the FJTL having the 20 JJs. The FJTL develops a voltage (at 5GHz, this voltage is approximately 10 μV) which then drives a 100 μAcurrent through the resistor 118 with a characteristic time of t=L/R=1.3ns. At time t=50 ns, the RF drive frequency input through RF input port104 is changed to 10 GHz and the resulting current driven through theresistor 118 is doubled. Finally, at time t=100 ns, the RF drivefrequency is set again to 5 GHz resulting in the current driven throughthe resistor 118 returning to the original 100 μA value.

FIG. 4 is a block diagram of a quantum mechanical system 200 includingthe RF to DC converter 100, according to an embodiment of the presentinvention. The quantum mechanical system 200 includes the RF to DCconverter 100 which may be provided with one or more Josephson junctionsJJs 108A, 108B, 108C, 108D, for example. In an embodiment, the quantummechanical system 200 also includes at least one quantum mechanicaldevice 202 connected to the resistor 118 of the RF to DC converter 100.In an embodiment, the one or more devices 202 includes at least one of aqubit, a superconducting quantum interference device, or a non-quantummechanical circuit such as, for example, a transistor or other circuit.

In an embodiment, it may be beneficial to be able to increase theoperating voltage of the FJTL. In order to increase the operatingvoltage, the quantum mechanical system 200 may include one or more rapidsingle flux quantum (RSFQ) pulse doubler stages 204. The one or moreRSFQ pulse doubler stages 204 can be provided prior to the RF to DCconverter 100 so as to achieve a gain of 2^(m) increase in operatingvoltage, where m is the number of RSFQ pulse doubler stages.

FIG. 5 is a schematic electronic circuit of an example conventionalrapid single flux quantum (RSFQ) pulse doubler 204. The RSFQ pulsedoubler 204 has an input port 502 and an output port 504. The outputport 504 of the RSFQ pulse doubler 204 is connected to an input port 104of the DC to SFQ converter 100. The RSFQ pulse doubler 204 is configuredto generate single flux quantum (SFQ) pulse current input through theinput port 104 of the DC to SFQ converter 100. However, other types ofRSFQ pulse doublers can also be used.

FIG. 6 is a plot of a single pulse input into the RSFQ pulse doubler 204and a plurality of pulses output by the RSFQ pulse doubler 204,according to an embodiment of the present invention. In an embodiment,the RSFQ pulse doubler 204 is configured to generate a plurality ofcurrent pulses 604 at double a rate of applied SFQ pulses at the outputport of the RSFQ pulse doubler 204 from a single radiofrequency currentpulse 602 input at the input port of RSFQ pulse doubler 204 so as togenerate a larger voltage across the converter 100. In an embodiment, mstages of the rapid single flux quantum (RSFQ) pulse doubler 204connected in series can be used. For example, FIG. 6 shows the result ofa simulation (for example, using WRSpice) of the RSFQ pulse doubler 204shown in FIG. 5 cascaded 5 times in series. The m stages of the RSFQpulse doubler 204 can be configured to generate the RF electricalcurrent (I) that is given by the following formula:

I=2^(m)×Φ₀ ×f _(clk) /R

where Φ₀ is a superconducting magnetic flux quantum, f_(clk) is the RFfrequency of the RF electrical current, and R is a resistance value ofthe resistor.

In an embodiment, when an input pulse arrives at the input port 502,Josephson junctions J1 and J2 switch sequentially. The SFQ pulse from J2splits between the upper path formed from Josephson junctions J4 and J5and the lower branch formed from Josephson junctions J2 and J3. Theshunt resistor Rs in the lower branch sets an L/R rise time of thecurrent in the lower branch delaying any switching action in Josephsonjunction J3. The upper branch pulse switches Josephson junction J5 whichdelivers a pulse to the output port 504 while also driving current into

Josephson junctions J3 and J4. This additional current along with thatfrom the delayed current from the switching of Josephson junction J2forces Josephson junction J3 to switch and produce a second pulse at theoutput. Josephson junction J4 acts as a protection junction preventingJosephson junction J2 from switching twice. For m stages placed inseries, the resulting current driven by the RF to DC converter 100 isthen I=2^(m)×Φ₀×f_(clk)/R. This circuit and it's operation is frompre-existing literature. Does that matter?

FIG. 7 is schematic diagram depicting how a single dc/SFQ converter 106can provide controllably fast flux bias to a plurality of devices (DUTs)via a plurality of RF to DC converters, according to an embodiment ofthe present invention. In an embodiment, the quantum mechanical system200 includes a plurality of radio-frequency (RF) to direct current (DC)converters 100. The quantum mechanical system 200 also includes aplurality of devices DUTs (e.g., quantum mechanical devices) 202. Eachof the plurality of quantum mechanical devices 202 is connected to acorresponding one of the plurality of RF to DC converters 100. Theplurality of RF to DC converters 100 are addressable so that SFQ pulsesproduced from the RF electrical current by the dc to SFQ converter 106are routed to a desired converter in the plurality of RF to DCconverters (FJTLs) 100.

In an embodiment, the quantum mechanical system 200 further includes aninput port 702 configured to receive the direct electrical current (DC)and the radiofrequency (RF) electrical current; and a plurality ofaddress lines 704, each address line 704 having at least onedemultiplexer (DEMUX) 706. A demultiplexer (DEMUX) 706A in a firstaddress line 704A is connected to the input port 702 via the DC/SFQconverter 106. Each 100 converter in the plurality of radio-frequency(RF) to direct current (DC) converters (FJTLs) 100 is connected to acorresponding one of the least one demultiplexer (DEMUX) 706.

In an embodiment, the demultiplexer (DEMUX) 706A in the first addressline 704A is connected to two demultiplexers (DEMUX) 706 in a secondaddress line 704B and each of the two demultiplexers 706 is connected toat least two radio-frequency (RF) to direct current (DC) converters(FJTLs) 100.

Therefore, in an embodiment, a series combination of multistage RSFQpulse multipliers 204 and resistively shunted FJTL 100 can be placed atan end of a RSFQ DEMUX tree such that a single dc/SFQ converter source106 can drive a multitude of devices (DUTs) 202. As shown in FIG. 7, asingle DC/SFQ converter 106 drives the input of a flux biased 1:2 DEMUX706. Depending on the sign of the current in the respective address line704, the flux pulse from the DC/SFQ converter 106 is routed either tothe left or the right of the 1:2 DEMUX 704.

FIG. 8 is a schematic electronic circuit of a bipolar quantizedsupercurrent generator (QSG) 800, according to an embodiment of thepresent invention. The bipolar quantized supercurrent generator (QSG)800 includes a first input port 802 configured to receive at least oneincrement single flux quantum pulses and a second input port 804configured to receive at least one decrement single flux quantum pulses.The QSG 800 further includes a first Josephson junction (JJ) 806connected to the first input port 802 and a second Josephson junction(JJ) 808 connected to the second input port 804. The first and secondJosephson junctions 806, 808 are further connected to ground 809. TheQSG 800 also includes an inductor (L_(q)) 810 connected to the firstJosephson junction 806 and to the second Josephson junction 808. Theinductor (L_(q)) 810 and the first and second Josephson junctions (JJs)806 and 808 form a superconducting quantum interference device (SQUID)loop. In operation, an electrical current circulating in the storageSQUID loop formed by the first Josephson junction 806, the secondJosephson junction 808 and the inductor (L_(q)) 810 increases ordecreases in increments based on the at least one increment single fluxquantum pulses input through the first input port 802 or the at leastone decrement single flux quantum pulses input through the second inputport 804, respectively.

In an embodiment, the electrical current circulating in the storageSQUID loop increases or decreases by electrical current increments AIgiven by the following equation:

ΔI=Φ ₀ /L _(q)

where Φ₀ is a superconducting magnetic flux quantum and L_(q) is aninductance value of the inductor (L_(q)).

In an embodiment, the QSG 800 also includes a third input port 813connected to the first Josephson junction (JJ) 806, the second Josephsonjunction (JJ) 808 and the inductor (Lq) 810. The third input port 813 isconfigured to input a bias direct electrical current (DC) to the storageloop to electrically bias the first and second Josephson junctions 806and 808 so that the first and second Josephson junctions 806 and 808produce pulses when pulses are applied to their respective inputs.

In an embodiment, a screening parameter β_(L) of the SQUID loop isdependent on a critical electrical current I of the first and secondJosephson junctions 806 and 808 in the SQUID loop and the inductancevalue of the inductor connecting the first and second Josephsonjunctions.

In an embodiment, QSG 800 further includes a third Josephson junction(JJ) 812 connected to the first Josephson junction (JJ) 806 and thefirst input port 802 via a first induction line 816. QSG 800 alsoincludes a fourth Josephson junction (JJ) 814 connected to the secondJosephson junction (JJ) 808 and the second input port 804 via a secondinduction line 818. The first Josephson junction (JJ) 806 and the thirdJosephson junction (JJ) 812 form a first Josephson transmission line(JTL) 819 and the second Josephson junction (JJ) 808 and the fourthJosephson junction (JJ) 814 form a second Josephson transmission line(JTL) 820.

In an embodiment, when an SFQ pulse arrives at the first input port“Inc” 802, it triggers junctions J1 812 and J2 806 to switch, setting upa circulating current flowing from J2 812 through Lq 810 (left-to-rightarrow). The user can unset this circulating current by triggeringjunctions J3 808 and J4 814 from the second input port “Dec” 804 tocirculate current flowing from J3 808 through Lq 810 (right-to-leftarrow), resetting the device. Because the induction value L_(q) (andhence β₁) are so large, the circulating current is not large enough toover or under bias either junctions J3 808 or J2 806, allowing multiplepulses to be applied consecutively from either the first input port“Inc” 802 or the second input port “Dec” 804. SFQ pulses loaded from thefirst input port 802 or the second input port 804 increase or decrease,respectively, the current in the storage loop formed by L_(q) andjunctions J2 806 and J3 808 in increments of ΔI=Φ₀/L_(q). In anembodiment, β_(L) is equal to approximately 100.

Therefore, the storage inductor L_(q) in combination with the last stageJJs (in this case J2 806 and J3 808) of the first Josephson transmissionline (JTL) 819 and the second Josephson transmission line (JTL) 820,respectively, forms a superconducting quantum interference device(SQUID) with a β_(L) for example equal to about 100. This allows for thestorage of a multitude of magnetic flux quanta that can be loaded fromeither JTL (the first JTL 819 or the second JTL 820) resulting inprecise increments or decrements of current in L_(q) with step sizeΔI=Φ₀/L_(q). Once the appropriate amount of current is loaded into L_(q)as determined by the user, the circuit can be powered down with the fluxperpetually stored in the loop. In an embodiment, QSG 800 can support100-1000's of flux quanta worth of circulating current depending on thevalue of L_(q), thus allowing for precise steps in flux bias on theorder of 0.01-0.001 Φ₀.

FIGS. 9A-9D show dynamic simulations of the bipolar quantizedsupercurrent generator (QSG) 800, shown in FIG. 8, according to anembodiment of the present invention. As pulses, shown in FIG. 9A, areloaded from the second input port “Dec” 804 input the current I(L_(q))in L_(q) is lowered in quantized amounts Φ₀/L_(q), as shown in FIG. 9B.As pulses, shown in FIG. 9C, are loaded into the first input port “Inc”802, the current is increased in the same quantized amount per fluxpulse, as shown in FIG. 9D (ascending steps from left-to-right). SFQpulses applied to the first input port “Inc” 802 increase the storedcurrent in the loop in quantized amounts. The transition from one stepto a next step occurs upon application of a pulse. SFQ pulses, shown inFIG. 9C, applied to the second input port 804 lower the current storedin the quantizing inductance L_(q) in quantized amounts (descendingsteps from left-to-right), as shown in FIG. 9D.

FIG. 10 is a schematic diagram of a quantum mechanical system 1000 usingthe QSG 800 shown in FIG. 8, according to an embodiment of the presentinvention. The quantum mechanical system 1000 includes the bipolarquantized supercurrent generator (QSG) 800. In an embodiment, thequantum mechanical system includes a plurality of QSGs 800. In anembodiment, the quantum mechanical system further includes a pluralityof quantum mechanical devices 1002. Each of the plurality of quantummechanical devices 1002 is inductively coupled, for example viainductance 1004, to a corresponding one of the plurality of QSGs 800.The plurality of QSGs 800 are addressable so that input SFQ pulses, fromDC/SFQ converter 1006, are routed to a desired QSG in the plurality ofQSGs 800.

In an embodiment, the quantum mechanical system 1000 further includes aninput port 1008 configured to receive a direct electrical current (DC)and a single flux quantum (SFQ) radiofrequency current. The quantummechanical system 1000 further includes a plurality of address lines1010. Each address line 1010 has at least one demultiplexer (DEMUX)1012. A demultiplexer (DEMUX) 1012A in a first address line 1010A isconnected to the input port 1008. Each QSG 800 is connected to acorresponding one of the least one demultiplexer (DEMUX) 1012.

In an embodiment, a single DC/SFQ converter 1006 can provide bi-polarflux bias to numerous different qubits 1002. The polarity of the currenton the address lines 1010 determines whether the SFQ pulse, from theDC/SFQ converter 1006, is routed to the left or right output of the 1:2DEMUX 1012. At the final stage, the polarity of the current on theaddress line determines whether the pulse is delivered to the firstinput port “Inc” 802 or the second input port “Dec” 804 of thecorresponding QSG 800. When the QSG 800 is coupled to the SQUID loop ofa superconducting qubit (QB) 1002, this can provide both positive andnegative flux bias. The scaling in the number of devices (e.g., qubits)that are able to be biased in such an architecture with the number ofaddress lines is 2^((n−1)), where n is the number of address lines.

One benefit of using the present QSG 800 is to provide the ability togenerate bi-polar persistent current with zero quiescent powerdissipation. This opens the door to efficiently flux bias circuitsinside cryostats at any temperature stage. In addition, when combinedwith the added benefit of DEMUXing control signals, the QSGs togetherwith the DEMUX configuration shown in FIG. 10 provides a scalable pathto DC flux biasing quantum processors with 1000's of devices (e.g.,qubits) with an exponential decrease in the used number of control linesfrom room temperature.

The descriptions of the various embodiments of the present inventionhave been presented for purposes of illustration, but are not intendedto be exhaustive or limited to the embodiments disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the describedembodiments. The terminology used herein was chosen to best explain theprinciples of the embodiments, the practical application or technicalimprovement over technologies found in the marketplace, or to enableothers of ordinary skill in the art to understand the embodimentsdisclosed herein.

We claim:
 1. A radio-frequency (RF) to direct current (DC) convertercomprising: a direct electrical current (DC) input port; aradiofrequency (RF) input port; a direct electrical current (DC) tosingle flux quantum (SFQ) converter connected to the RF input port, theDC to SFQ converter being configured to convert RF current to SFQcurrent pulses; a Josephson junction (JJ) connected to both the DC inputport via a first induction line and to the DC to SFQ converter via asecond induction line and to ground; and a resistor connected to theJosephson junction and to the DC input port via a third induction line,wherein, in operation, when a DC electrical current is applied via theDC input port, no DC electrical current flows through the resistor, andwhen an RF electrical current at an RF frequency is applied via the RFinput port, an electrical current directly dependent on the RF frequencyflows through the resistor based on the SFQ current pulses from the DCto SFQ converter switching the JJ.
 2. The converter according to claim1, further comprising: a plurality of Josephson junctions connected toboth the DC input port via the first induction line and to the RF inputport via the DC to SFQ converter through the second induction line andto ground, wherein the plurality of Josephson junctions are configuredto switch, when the RF electrical current is applied via the RF inputport, at a rate commensurate the RF frequency of the RF electricalcurrent to generate a steady state voltage across the plurality ofJosephson junctions (JJ) linearly dependent on the RF frequency suchthat the electrical current flowing through the resistor is directlydependent on the RF frequency of the RF electrical current.
 3. Theconverter according to claim 1, wherein the steady state voltage (V)across the Josephson junction (JJ) is proportional to the RF frequency(folk) according to the following equation:V=Φ ₀ ×f _(clk) where Φ₀ is a superconducting magnetic flux quantum. 4.A quantum mechanical system comprising: a radio-frequency (RF) to directcurrent (DC) converter comprising: a direct electrical current (DC)input port; a radiofrequency (RF) input port; a direct electricalcurrent (DC) to single flux quantum (SFQ) converter connected to the RFinput port, the DC to SFQ converter being configured to convert RFcurrent to SFQ current pulses; a Josephson junction (JJ) connected toboth the DC input port via a first induction line and to the DC to SFQconverter via a second induction line and to ground; and a resistorconnected to the Josephson junction and to the DC input port via a thirdinduction line, wherein, in operation, when a DC electrical current isapplied via the DC input port, the DC electrical current is shunted toground through the Josephson junction (JJ) and substantially noelectrical current flows through the resistor, and when an RF electricalcurrent is applied via the RF input port, the Josephson junction (JJ)switches at a rate commensurate an RF frequency of the RF electricalcurrent to generate a steady state voltage across the Josephson junction(JJ) linearly dependent on the RF frequency such that an electricalcurrent flowing through the resistor is directly dependent on the RFfrequency of the RF electrical current.
 5. The quantum mechanical systemaccording to claim 4, further comprising at least one quantum mechanicaldevice connected to the resistor.
 6. The quantum mechanical systemaccording to claim 5, wherein the one or more devices includes at leastone of a qubit, a superconducting quantum interference device, or anon-quantum mechanical circuit.
 7. The quantum mechanical systemaccording to claim 4, further comprising: a rapid single flux quantum(RSFQ) pulse doubler having an input port and an output port, whereinthe output port of the RSFQ pulse doubler is connected to an input portof the DC to SFQ converter, the RSFQ pulse doubler being configured togenerate SFQ pulse current input through the input port of the DC to SFQconverter.
 8. The quantum mechanical system according to claim 7,wherein the RSFQ pulse doubler is configured to generate a plurality ofcurrent pulses at double a rate of applied SFQ pulses at the output portof the RSFQ pulse doubler from a single radiofrequency current pulseinput at the input port of RSFQ pulse doubler so as to generate a largervoltage across the converter.
 9. The quantum mechanical system accordingto claim 7, further comprising: m stages of the rapid single fluxquantum (RSFQ) pulse doubler connected in series, wherein the m stagesof the RSFQ pulse doubler are configured to generate the RF electricalcurrent (I) that is given by the following formula:I=2^(m)×Φ₀ ×f _(clk) /R where Φ₀ is a superconducting magnetic fluxquantum, f_(clk) is the RF frequency of the RF electrical current, and Ris a resistance value of the resistor.
 10. The quantum mechanical systemaccording to claim 4, further comprising: a plurality of radio-frequency(RF) to direct current (DC) converters; and a plurality of quantummechanical devices, each of the plurality of quantum mechanical devicesbeing connected to a corresponding one of the plurality of RF to DCconverters, wherein the plurality of RF to DC converters are addressableso that the SFQ pulses produced from the RF electrical current arerouted to a desired converter in the plurality of RF to DC converters.11. The quantum mechanical system according to claim 10, furthercomprising: an input port configured to receive the direct electricalcurrent (DC) and the radiofrequency (RF) electrical current; and aplurality of address lines, each address line having at least onedemultiplexer (DEMUX) and a demultiplexer (DEMUX) in a first addressline is connected to the input port, wherein each converter in theplurality of radio-frequency (RF) to direct current (DC) converters isconnected to a corresponding one of the least one demultiplexer (DEMUX).12. The quantum mechanical system according to claim 11, wherein thedemultiplexer (DEMUX) in the first address line is connected to twodemultiplexers (DEMUX) in a second address line and each of the twodemultiplexers is connected to at least two radio-frequency (RF) todirect current (DC) converters.
 13. A bipolar quantized supercurrentgenerator (QSG) comprising: a first input port configured to receive atleast one increment single flux quantum pulses and a second input portconfigured to receive at least one decrement single flux quantum pulses;a first Josephson junction (JJ) connected to the first input port and asecond Josephson junction (JJ) connected to the second input port, thefirst and second Josephson junctions being further connected to ground;and an inductor (L_(q)) connected to the first Josephson junction and tothe second Josephson junction, wherein the inductor (L_(q)) and thefirst and second Josephson junctions (JJs) form a superconductingquantum interference device (SQUID) loop, wherein in operation, anelectrical current circulating in the storage SQUID loop formed by thefirst Josephson junction, the second Josephson junction and the inductor(L_(q)) increases or decreases in increments based on the at least oneincrement single flux quantum pulses input through the first input portor the at least one decrement single flux quantum pulses input throughthe second input port, respectively.
 14. The bipolar quantizedsupercurrent generator (QSG) according to claim 13, wherein theelectrical current circulating in the storage SQUID loop increases ordecreases by electrical current increments AI given by the followingequation:ΔI=Φ ₀ /L _(q) where Φ₀ is a superconducting magnetic flux quantum andL_(q) is an inductance value of the inductor (L_(q)).
 15. The bipolarquantized supercurrent generator (QSG) according to claim 13, furthercomprising a third input port connected to the first Josephson junction(JJ), the second Josephson junction (JJ) and the inductor (Lq), thethird input port being configured to input a bias direct electricalcurrent (DC) to the storage loop to electrically bias the first andsecond Josephson junctions so that the first and second Josephsonjunctions produce pulses when pulses are applied to their respectiveinputs.
 16. The bipolar quantized supercurrent generator (QSG) accordingto claim 13, wherein a screening parameter β_(L) of the SQUID loop isdependent on a critical electrical current I of the first and secondJosephson junctions in the SQUID loop and the inductance value of theinductor connecting the first and second Josephson junctions.
 17. Thebipolar quantized supercurrent generator (QSG) according to claim 13,further comprising: a third Josephson junction (JJ) connected to thefirst Josephson junction (JJ) and the first input port via a firstinduction line; and a fourth Josephson junction (JJ) connected to thesecond Josephson junction (JJ) and the second input port via a secondinduction line, wherein the first Josephson junction (JJ) and the thirdJosephson junction (JJ) form a first Josephson transmission line (JTL)and the second Josephson junction (JJ) and the fourth Josephson junction(JJ) form a second Josephson transmission line (JTL).
 18. A quantummechanical system comprising: a bipolar quantized supercurrent generator(QSG) comprising: a first input port configured to receive one or moreincrement single flux quantum pulses and a second input port configuredto receive one or more decrement single flux quantum pulses; a firstJosephson junction (JJ) connected to the first input port and a secondJosephson junction (JJ) connected to the second input port, the firstand second Josephson junctions being further connected to ground; and aninductor (L_(q)) connected to the first Josephson junction and to thesecond Josephson junction, wherein the inductor (L_(q)) and the firstand second Josephson junctions (JJs) form a superconducting quantuminterference device (SQUID) loop, wherein in operation, an electricalcurrent circulating in the storage SQUID loop formed by the firstJosephson junction, the second Josephson junction and the inductor(L_(q)) increases or decreases in increments based on the one or moreincrement single flux quantum pulses input through the first input portor the one or more decrement single flux quantum pulses input throughthe second input port, respectively.
 19. The quantum mechanical systemaccording to claim 18, further comprising: a plurality of bipolarquantized supercurrent generators (QSGs); and a plurality of quantummechanical devices, each of the plurality of quantum mechanical devicesbeing inductively coupled to a corresponding one of the plurality ofQSGs, wherein the plurality of QSGs are addressable so that input SFQpulses are routed to a desired QSG in the plurality of QSGs.
 20. Thequantum mechanical system according to claim 19, further comprising: aninput port configured to receive a direct electrical current (DC) and asingle flux quantum (SFQ) radiofrequency current; a plurality of addresslines, each address line having at least one demultiplexer (DEMUX) ademultiplexer (DEMUX) in a first address line is connected to the inputport, wherein each QSG is connected to a corresponding one of the leastone demultiplexer (DEMUX).